There are various applications where it is necessary to selectively focus an optical beam on a surface which may be at any of a wide range of distances from a reference point in the optical system. Conventionally, this is done through the intermediacy of a series of positionable lenses as, for example, in a slide or movie projector. Such an optical system requires accurately made lenses and requires a rather complex mechanical linkage system for moving the lenses relative to one another, which must be capable of holding the lenses once positioned. Additionally, problems exist in providing a proper mechanical linkage if the focus of the system must be rapidly and/or frequently changed. Such requirements are present when an optical beam is to be focused at the surface of a series of articles of different sizes rapidly passing through a reading station where each article is optically scanned.